ES2391716T3 - Direct power and stator flow vector control of a generator for a wind energy conversion system - Google Patents

Abstract

Method for controlling a variable speed wind turbine generator (11) connected to a power converter (13) comprising switches (52, 53), said generator comprising a stator and a set of determinals connected to said stator and to said switches, said comprising procedure: determine a reference stator flow value corresponding to a power generator (11) of a desired magnitude, determine an estimated stator flow value corresponding to a real generator power (11), determine a flow difference value of the stator between the reference value of the determined stator flow and the estimated stator flow value, and operating said switches (52, 53) according to a modulation scheme of the space vector to control a switching pattern of said switches (52, 53) , wherein said switching pattern is formed by the application of one or more vectors for one or more periods of switching, and said switching periods for the switching pattern are determined from the magnitude and direction of the stator flux difference vector to adapt at least an electrical amount of a stator to obtain the power magnitude of said generator (11) wanted.

Description

Direct power and stator flow vector control of a generator for a wind energy conversion system

Technical field

The present invention relates, in general, to power converters, and more specifically to power converters that can be connected to wind turbine generators (WTG) operating at variable speeds, thus providing a voltage and a current with an amplitude and a variable frequency

Background of the invention

Wind has long been used as a source of energy and, in recent years it has become very common to use wind to produce electrical power. In order to do this, wind energy is captured by a set of blades (usually two or three) from a wind power plant. The wind captured by the blades causes an axis connected to the blades to rotate. The shaft is connected to a rotor of a generator, which, therefore, rotates at the same speed as the shaft, or at a multiple of the shaft speed in case the rotor is connected to the shaft through a box of gears Next, the generator converts the mechanical power provided by the wind into electrical power for supply to a network.

To optimize the efficiency of a wind turbine generator, the use of a variable speed generator is preferred, in which the speed of the blades and, therefore, the speed of the shaft, will depend on the wind speed. This implies that an optimal operating point for the WTG must be established at different wind speeds. This is done by controlling the torque and the active (real) power supplied by the generator.

The main purpose of a WTG is to provide active power. Active power is the component of the total or apparent electrical power that performs a job and is measured in watts. The control system in a WTG will control the active power extracted from the WTG to track the optimum operating speed point for the WTG, using torque control or power control.

When power control is used, a power command based on an estimate of wind power is supplied to the control system. This commanded value is compared with the output power of the real WTG and the difference is controlled.

When torque control is used, a torque command based on an available torque from the shaft is supplied to the control system. This commanded value is compared with the actual generator torque and the difference is controlled.

The reactive power, measured in volts-amps, establishes and sustains the electric and magnetic fields of alternating current machines. The apparent power, measured in volts-amps, is the vector sum of the active and reactive powers. Modern WTG control systems can control both active and reactivated power.

A first type of control systems for WTG is related to the independent control of (normally) three spatially displaced sinusoidal voltages 120 ° of the three phases of the generator stator. The generation of sine waves is based on the properties of the generator, that is, an equivalent model for the generator when operating under permanent conditions is derived from the electrical and mechanical characteristics of the generator, in which the control system is designed based on the type of generator used (for example, asynchronous or synchronous).

The generation of one of the sine waves in the three-phase system is normally carried out independently of the other sine waves, that is, this type of control system operates as a three-control system of a separate single-phase system instead of as a common three phase control. This fact results in that any imbalance in the three-phase system or any interaction between the phases will not be considered in this type of control. On the other hand, it is clear that the generator model will only be valid during steady state operation of the generator. During the transient operation of the generator (start, stop, load changes, etc.) the control will therefore allow a high peak of voltage and transient current. This results in decreased power conversion efficiency, as well as a need to oversize the electrical components of the WTG system to cope with transient overvoltage currents and voltages.

To overcome the drawbacks of the previous control structure, an alternative control structure generally called field oriented control (FOC) has been introduced. The main idea behind the FOC is the control of the generator stator currents through the use of a vector representation of the currents. More specifically, the FOC relies on the transformation of coordinates that transform a three-phase time and a speed-dependent system into a two-coordinate invariant time system.

The advantage of performing a transformation from a three-phase stationary coordinate system to a rotation coordinate system is that the generator control can be performed by controlling the

quantities of CC and the response to transients is improved compared to that achieved with the independent control of the three phases.

The FOC transformation is carried out in two stages: 1) transformation of the three-phase stationary coordinate system abc to one of two phases, called the αβ stationary coordinate system (known as Clarke's transformation), and 2) transformation of the coordinate system stationary αβ to a rotation coordinate system dq (known as Park transformation). More specifically, the transformation of the natural abc reference frame to the synchronous dq reference frame is obtained by the equations

Y

what it provides

where θ = ωt is the angle between the stationary α axis and the synchronous d axis.

The control of a generator by means of the FOC requires a flow component aligned with the d axis. As explained above, the dyq oriented components are transformations of the stationary three-phase coordinate system that implies that the FOC, due to the direct coupling to the three-phase electrical quantities, will be responsible for the stationary state and the transient operation of the system regardless of the generator model.

An advantage of the FOC is that the control of the parameters in rotating coordinates theoretically allows a decoupled control between the parameters. Therefore, with the amplitude of the rotor flow controlled at a fixed value and the linear relationship between the torque and the torque component iq of the stator it is possible to achieve a satisfactory control decoupled from the output power control of the WTG.

US5083039 discloses a variable speed wind turbine comprising a turbine rotor that drives a multi-phase generator, a power converter with switches that control electrical quantities of the stator in each phase of the generator, a torque control device associated with turbine parameter sensors that generate a torque reference signal indicative of a desired torque, and an operating generator controller under the control of the field orientation and sensitive to the torque reference signal to define an axis current of desired quadrature and to control the switches to produce electrical quantities of the stator corresponding to the desired quadrature current axis.

The power control of the FOC-based generator is also described in other documents, for example JP 2002276533, GB 2411252, US 6420795 and US 2007278797.

Despite the advantages with the FOC described above, there are deficiencies of the conventional controllers with which the industry has lived. These include, for example (a) a difficulty in maintaining the correct decoupling between the flow and the torque produced by components of the stator currents during the equilibrium and dynamic state, (b) the control of the currents using linear controllers a higher speeds and higher modulation index. Case (a) refers to the sensitivity of parameters and the need to adapt them. This can put the controller reliability in tension under extreme load conditions. Case (b), on the other hand, refers to the underutilization of the CC link. In addition, since both procedures described above can be used for the control of wind turbine output, and the output control ultimately involves an interaction between the flows or currents and flows, an ability to directly control the flow leads to systems more robust and simpler.

Summary of the invention

In view of the foregoing, an object of the invention is to provide an alternative method for independent control of the three phases, as well as the classic FOC for controlling the power output of a WTG.

In particular, one objective is to provide a procedure for direct flow control in a design that has a greater control range for expected DC link voltages and in which the stator switches are directly operated on the basis of a flow of the desired stator in a stationary frame of reference. The objective, furthermore, is to provide a method for determining the optimal switching times for a spatial vector modulation scheme using an error signal from the stator flow vector.

These objectives are met by the characteristics of the method of claim 1 and the apparatus of claim 6. The dependent claims describe advantageous embodiments and details of the invention.

In general, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless expressly defined otherwise herein. All references to "a / one / the [element, device, component, medium, stage, etc.]" should be openly interpreted as a reference to at least one instance of said element, device, component, medium, stage, etc., unless explicitly stated otherwise. The steps of any procedure described in this document do not have to be carried out in the exact order described, unless explicitly stated.

Brief description of the drawings

The foregoing, as well as other objectives, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the accompanying drawings, where the same numerical reference It is used for similar elements, in which:

Figure 1 illustrates the generator converter system according to a preferred embodiment of the present invention.

Figure 2 illustrates a vector diagram for a synchronous generator represented in a stationary reference frame.

Figure 3 illustrates a vector representation of the voltages present in the stator terminals of a generator.

Figure 4a is a more detailed illustration of the generator side converter illustrated in Figure 1.

Figure 4b illustrates eight switching states that determine a space vector hexagon.

Figure 5a illustrates a sector of the space vector hexagon illustrated in Figure 4b.

Figure 5b illustrates a normalized tension vector.

Figure 6 illustrates a control system for controlling the power of a wind turbine generator in accordance with an embodiment of the present invention.

Figures 7a-c illustrate a signal flow graph of a power generator and stator flow vector controller according to an embodiment of the invention.

Figure 8 illustrates a graph to achieve predictive control to mitigate a phase error in the stator flow vector.

Figure 9 illustrates a principle of current limitation in the flow vector generation of the reference stator.

Figure 10 illustrates a field weakening decision flow diagram.

Detailed description of the preferred embodiments

Figure 1 illustrates an example of a generator converter system according to a preferred embodiment of the present invention.

An axis 10 transfers the mechanical power from a power source, preferably a set of blades of

wind turbine (not shown), to a rotor of a variable speed generator 11. The shaft is preferably connected to the blades of the wind turbine, and to the rotor through a gearbox to adapt the rotation speed of the shaft 10 (ie, speed of the wind turbine blades) at a suitable speed range for the generator 11. The generator 11 converts the mechanical power provided through the axis 10 into electrical power and supplies the electrical power to a set of terminals of the stator 12a, 12b, 12c. For optimum performance with respect to the conversion of wind power into electric power, axis 10 will vary its speed as a function of wind speed. Since the rotation speed of the generator rotor 11 is proportional to the rotation speed of the shaft 10, the amplitude and frequency of the voltage signal provided by the generator 11 to the stator terminals 12a, 12b, 12c will vary according with the rotation speed of the axis 10. The generator can be a single or double fed synchronous generator, for example, a permanent magnet generator (PM), an induction generator or any other type of generator comprising a stator winding.

The terminals 12a, 12b, 12c of the generator 11 are connected to a power converter on the generator side

13. The converter 13 is preferably a three-phase bridge converter 13 that includes six switches illustrated for reasons of clarity by the single switch and the diode of Figure 1. As will be described in more detail below, the switches are arranged in a set of upper and lower switches that are preferably in the form of solid state devices, such as MOSFET, IGBT or GTO. Other types of switches, such as BJT, however, are equally possible depending on the design considerations of the converter 13. The converter 13 will operate under normal operation as an active rectifier that converts the variable frequency of AC voltage supplied by the generator 11 in a DC voltage. The conversion is controlled with a pulse width modulation scheme, in which control signals are applied to the switches in the converter 13 to provide the desired conversion functionality. In a preferred embodiment, the switches are controlled by the use of the space vector modulation scheme, as described below.

The output of the converter 13 is provided to a DC link 14, which comprises a link capacitor to reduce the voltage ripple in the DC circuit.

The DC link 14 is connected to a network side power converter 15. The topology of the network side power converter 15 is similar to the generator side power converter 13 described above. The power converter on the network side 15 normally functions as an inverter to convert the DC voltage on the DC link 14 into a regulated DC voltage for the active and reactive power supply to the network 18. The inverter switches of power on the side of the network 15 are provided with the appropriate control voltages to provide the desired voltage and power to a network 18.

The output from the power converter on the network side 15 is filtered by inductors 16a, 16b, 16c in order, for example, to eliminate higher order harmonics from the output power signal. The output power signal is then provided to the network 18 through a transformer 19. The output power signal can, if necessary, be filtered by a second filter 17 in order to maintain interference or distortion. Harmonic at a low value.

Figure 2 illustrates a vector diagram for a synchronous generator represented in a stationary reference frame. The diagram comprises two stationary axes indicated α and β. The transformation from the abc reference frame of three stationary phases to the αβ reference frame can be carried out as described above.

A first vector represents the magnetization flow, indicated Ψmag in the figure. In the example shown in Figure 2, which refers to a synchronous generator, the magnetization flow corresponds to the rotor flow. The rotor flow can be generated by means of a permanent magnet, as in a PM generator, or by the excitation of a field coil in the rotor. The arc at the end of the rotor flow vector illustrates that the vector revolves around the origin of coordinates in the figure. The angular displacement of the rotor flow vector from the α axis is indicated as θr in the figure.

In a corresponding way is the stator flow vector, indicated Ψs in the figure, represented by a vector that rotates around the origin of the coordinates. In steady state operation the stator flow vector rotates in the stationary reference frame with an angular velocity equal to the rotor flow vector. The angular displacement of the stator flow vector from the rotor flow vector is indicated by δ in the figure.

The electromagnetic power of a synchronous generator can be expressed as:

which provides 5

It can be seen from the previous power equation that for a given operating speed, the electromagnetic power depends on the magnitude of the stator flow vector and its location with respect to the rotor flow vector. If the position of the rotor flow vector is known, it is possible to apply a voltage that will position the stator flow vector to give the desired magnitude of power at a given speed. Therefore, by controlling the stator flow vector, the electromagnetic power (EM), which corresponds to the charging power, can be achieved as ordered.

Since the control is carried out in the stationary reference frame, it may be necessary to compensate for the phase delay created. This is achieved by a linear prediction carried out in polar coordinates.

Figure 3 illustrates a vector representation of the voltages present in the stator terminals of a generator. In order to control the power created by the generator it is necessary to control the signals that are applied to the stator terminals. In this regard, spatial vector modulation (SVM) is an effective average algorithm for providing an AC output signal from a DC voltage. The SVM also minimizes the harmonic content that determines the losses of copper in the generator. The SVM is also effective in minimizing switching losses in the power converter switches on the generator side 13.

For a three-phase generator, the voltages in the stationary reference frame abc can be represented as three phase vectors displaced 120 ° (directions ua, ub and uuc) in space, as shown in the figure

3. For a three-phase balancing system, these vectors add to zero. This implies that the three vectors can be represented by a single space reference vector (us). The idea behind the SVM is to control the amplitude and frequency of Vs, which implies that the amplitude of the phase voltage, and the frequency at the stator terminals 12a, b, c and therefore the flow in the stator can be controlled

Referring to Figure 4a, which is a more detailed illustration of the generator side converter 13 is shown in Figure 1. The switches 52ab, b, c and 53a, b, c in the figure are illustrated as BJT. It is, however, equally possible to use MOSFET, GTO, IGBT, etc. as switching devices. Regardless of the technology used for the manufacture of switches 52a, b, c, and 53a, b, c, the switching sequence, or the switching pattern, of the devices must follow certain rules. More specifically, each time one of the upper switches 52a, b, c is driving (ie, in a state of on) the corresponding lower switch 53a, b, c must be turned off, and vice versa. In addition, three of the switches must always be on and three switches must always be off. These rules give rise to eight different combinations for the switching states of devices 52a, b, c and 53a, b, c. These combinations are indicated (abc) where, for example a = 1, b = 0 and c = 0 indicates that the upper switch 52a is on (thus turning switch 53a off) while switches 52b and c are off. Six of the states are active states that produce a voltage vector in a predefined direction, while two of the states are inactive states, that is, all upper switches 52a, b, c are off and all lower switches 53a, b, c are on, or vice versa.

The eight switching states defined above determine eight phase voltage configurations as illustrated in Figure 4b. As seen in the figure, the vectors define a hexagon, with six sectors of equal size separated 60º. Each sector is delimited by two active vectors. Inactive states are represented by vectors (000) and (111) that are zero and are at the origin of the hexagon. Two adjacent voltage vectors are chosen based on the sector in which the us vector is located (100 and 110 in Figure 4b). From Figure 4b it is clear that only one of the upper and lower switches changes state when the change pattern moves from one sector to the adjacent sector, in which the switching losses are kept to a minimum.

Normally, the switches operate at a frequency F, which is substantially higher than the network frequency. The switching frequency F 15 defines the sampling period! S through the ratio! S = 1 / F. The sampling period! S is used when generating the vector Vs from the different voltage vectors (100, 110, etc.) Specifically, it is the vector us formed by the temporal weighting vectors during a sampling period! S. Mathematically the vector us can be expressed as

where! 0,! 1 ...! 7 is the time that each vector u0, u1 ... u7 is applied, respectively. The vectors u0 and u7 are the zero vectors (000, 111) that are applied in order to produce a zero voltage.

When us and! S are known, it is possible to determine the on time for each vector, respectively, at 6

from the equations

Y

5 A problem lies in how to determine the desired us vector to provide efficient control of the electrical power supplied by the generator.

Figure 5a illustrates a sector of the space vector hexagon shown in Figure 4b. The desired stator flow vector Ψs at two subsequent time points is illustrated as the vectors Ψs (k) and Ψs (k + 1). The flow reference Ψs * is represented by a circle arc in the figure. The difference between the flow vector of

10 desired stator and the reference flow creates a flow error vector ΔΨs * (k) with an address that is perpendicular to the desired flow direction. The flow in the stator refers to the generator EMF by the equation (Faraday's law)

This implies that the flow error vector ΔΨs * (k) is proportional to a tension vector that can be obtained

15 as an average in a sample using adjacent vectors and is displaced by an angle γ with respect to the tension vector ua (i.e., the active vectors u0, u1, etc.) in any activity sector. Therefore γ varies from 0 to 60 degrees in a sector. The time of each active vector, for example, u1 and u2 is applied in figure 5 is represented by! A and! B in the figure.

Figure 5b illustrates an example where a standardized voltage vector is used to generate the times of

20 switching The basis for normalization is taken as the peak value of the fundamental component of the phase voltage during six-stage operation

where UDC is the DC link voltage of a two-level inverter described above. In the spatial vector modulation scheme it can be shown that the length of each of the six vectors (u1-u6) is

in which the normalization of tension becomes Normalization = ∀ / 3 From figure 3 it can be seen that

30 from which the switching times for each of the active vectors can be derived, for example, which define the control signals that are applied to the power switches on the generator side of the converter 13, according to

where! a is the time at which the first vector is applied (for example, vector u1 in Figure 4b) and! b is the time at which the second vector is applied (for example, vector u2 in Figure 4b).

With a brief reference to Figures 1, 4a and 7, the determined switching times are used as control signals by a switching control unit indicated by the PWM block 82. The PWM block 82 uses the control signal to control switches 52, 53 on the generator side power converter

13. By changing the states of the switches in the power converter on the generator side 13 it is possible to establish electrical quantities of the stator such that a desired power generator level is reached. More specifically, the control signals cause the switches 52, 53 of the side power generator converter 13 to adjust the phase and magnitude of the voltage of their AC terminal voltage with respect to the EMF of the generator 11 in order to Provide the desired electrical power.

The voltage generated by the power converter on the side of the generator 13 is defined by the requirement of the flow controller. Thus the switching has to be carried out to mitigate the error in the stator flow vector ΔΨs * (k). This flow vector control approach can be extended to any modulation index. During the normal spatial vector modulation interval, the error can be compensated through switching in a sample.

Figure 6 illustrates a control system for controlling the power of a wind turbine generator in accordance with an embodiment of the present invention.

The power control command for the converter is compared with the estimation of the power supplied by the generator 71. The mechanical dynamics of the system being slower allows the power controller 79 to be used directly to give the reference of the flow vector of the stator

The generator 71 does not require reactive power unless at very high speeds when the field is weakened as necessary. The EM design caters to this aspect of the generator. Therefore, it is the active power requirement that drives the power controller in the slower generator dynamics. The stator flow vector is being controlled in a similar manner as explained above. The switching carried out using the stator flow vector error is the same as described above.

The generator controller will be described with the help of Figures 7a-c.

The basic operation of the controller is as follows: The inverter on the generator side receives the Pref power command based on either a difference of a desired generator speed and a real generator speed developed in partial load mode or a constant value over the basis of environmental conditions in full charge mode. The controller operates in a fixed system of coordinates of the generator stator.

The controller operates on the basic principle that the electrical power output of a generator is

where: ω is the speed of rotation of the generator;

is the magnitude of flow provided by the permanent magnet of the rotor

is the magnitude of the stator flow controlled by the PWM of the generator side inverter 8

sinγ is the sine of a desired angle γ between the flow generated by the permanent magnet of the rotor and the flow the stator controlled by the PWM of the generator side inverter

There are no operations to establish either a rotor current component or quadrature current component with a flow vector.

A power command, Pref, from the charge controllers (not shown) is fed to the error detector 501 and compared to the system output power provided for the common connection point. The output of the detector 501 is passed through the switch 502 to the controller Pl 504. The switch 502 is a protection device to eliminate the Pref command if, during a flow control process, the stator currents exceed a predetermined value such as It is detected in block 512.

The output of block 504 is fed through two gain blocks 505 that adjust the power signal as a function of the generator rotor speed and the stator current and converts the power error signal to a required flow ref ΨPEM_REF.

Ψ * PEM_REF is then compared in block 509, with an output flow value ΨPEM, which is proportional to the actual output power, to form a ΨPEM_REF_ERROR which after processing by the Pl 510 controller becomes Ψ * PEM Also Ψ * MAG_REF (from field weakening block 506) is compared with ΨMAG from block 520 to form a ΨMAG_ERROR which after processing by the Pl 508 controller and the addition of Ψr becomes Ψ * MAG. The logic contained in the field weakening block 506 is shown in Figure 10.

The values of ΨPEM and ΨMAG are calculated in blocks 520 and 522 using a total stator flow Ψs (determined in block 528) using the delta load angle (δ), the angle between a desired magnetization flow and the desired responsible flow of the power output, δ * load_ angle is determined in block 514 using the arctan (Ψ * PEM / Ψ * MAG), while the magnitude of the total stator flow | Ψ * s | It is also determined in block 514 as the square root of the sum of the squares of the flow components in block 514.

Up to this point, all signals have been DC values, there have been no transformations in rotation reference systems, nor has there been torque placement to produce quadrature current vectors with identified rotor flow vectors.

The stator flow vector, which has a magnitude and a position, is developed in block 516 and corrected in block 518 of the delays caused by the speed of rotation of the generator and the need to sample at discrete time intervals. More specifically, and as shown in Figure 8 the flow reference is adjusted by adjusting the time interval between k using the delta signal theta fed to block 518.

Field weakening is necessary when there is a gust of wind and the rotor speed exceeds the nominal speed. The generated EMF increases leaving a small voltage range for control of generator power and stator flow. It becomes essential to weaken the resulting air gap field, so that the generated EMF remains effective at a desired value even if there is an increase in speed. Figure 10 illustrates a flow chart of the field weakening procedure. The adjusted maximum modulation index is used to calculate the reference magnitude resulting from magnetization. Under conditions other than field weakening or low speed conditions, the calculated magnetization reference is greater than or equal to the magnitude of the rotor flow. At high speed, the calculation result will be applied. The procedure also has the possibility of "strengthening" the field in case the magnets weaken due to high temperatures.

Predictive control to mitigate the phase error of the stator flow vector is achieved as shown in Figure 8. The prediction is made in polar coordinates and generates the stator flow vector Ψps *. The estimated flow vector of the stator Ψs as shown is compared with the predicted reference stator flow vector and the error vector ΔΨs defines the switching states for the control of the active power and stator flow vector in the frame static reference.

The flow vector generation principle of the current limiting reference stator is shown in Figure 9. The fact that magnetization of a magnetized rotor machine such as surface mounted PM machine or a synchronous machine that feeds the rotor is not necessary , can be exploited to define the vector magnitude of the desired reference flow. Figure 9 illustrates this. The current vector needed in this control is only to meet the demand for active power and not the creation of any flow in the machine. Therefore, the magnitude of the minimum current vector that this requirement can achieve must be along a direction perpendicular to the rotor flow vector.

If the machine has to be used as an engine, the current vector must drive the rotor flow vector otherwise the rotor flow vector should be delayed as shown in the figure. Therefore, the reference flow vector component that contributes to the torque or active power can be derived directly with the location information of this current vector. This implies the entry of the rotor flow vector location, which is available from the incremental position and / or encoder attached to the machine axis. For generators with protrusions in the rotor structure, operation without sensors can be incorporated by measuring the voltage and currents thus eliminating the need for a speed / position sensor. The advantage is the possibility that the controller limits the current in the stationary reference frame. At very high operating speeds, it is possible to have a magnetization component of the

5 stator flow vector. This component may also be necessary when an indoor PM machine is used for power generation.

The invention has been described primarily above with reference to a few embodiments. However, as will be readily appreciated by a person skilled in the art, embodiments other than those described above are equally possible within the scope of the invention, as defined by the

10 patent claims attached.

Claims (11)

1. Method for controlling a variable speed wind turbine generator (11) connected to a power converter (13) comprising switches (52, 53), said generator comprising a stator and a set of terminals connected to said stator and said switches, said procedure comprising: determining a reference stator flow value corresponding to a power generator (11) of a desired magnitude, determine an estimated stator flow value corresponding to a real generator power (11), determine a stator flow difference value between the reference value of the determined stator flow and the estimated stator flow value, and operating said switches (52, 53) according to a modulation scheme of the space vector to control a switching pattern of said switches (52, 53), wherein said switching pattern is formed by the application of one or more vectors during one or more switching periods, and said switching periods for the switching pattern are determined from the magnitude and direction of the stator flux difference vector to adapt at least one electrical amount of a stator to obtain the power magnitude of said generator (11) desired.

2.

Method according to claim 1, wherein a stator flow difference vector with a magnitude and direction is determined by means of the difference between the reference value of the stator flow vector and the estimated value of the stator flow vector , and said switches (52, 53) operate based on said stator flow difference vector.

3.

Method according to any of claims 1 or 2, wherein said switches (52, 53) operate according to a pulse width modulation scheme to generate a voltage waveform synthesized at the stator terminals.

Four.

Method according to claim 1, wherein the switches (52, 53) comprise a first and a second set of switches (52, 53) and the first set of switches operates towards a state of ignition for a first time interval,! a, and the second set of switches to a state of on for a second time interval,! b.

5.

Method according to claim 4, wherein the first and second time intervals are determined according to

6.

Apparatus for controlling a variable speed wind turbine generator (11) connected to a power converter (13) comprising switches (52, 53), said generator (11) comprising a stator and a set of terminals connected to said stator and said switches, said apparatus comprising:

a power controller (79) adapted to determine a reference value of the stator flow corresponding to a generator power (11) of a desired magnitude, a flow estimator (78) adapted to determine an estimated stator flow value corresponding to a real power of the generator, a comparator adapted to determine a difference value of the stator flow between the determined reference value of the stator flow and the estimated value of stator flow, and a switch control unit (72) adapted to operate said switches (52, 53) according to a space vector modulation scheme to control a switching pattern of said switches (52, 53), wherein said control unit of the Switch (72) is adapted to determine the switching periods for the switching pattern from the magnitude and direction of the stator flow difference vector to adapt at least one electrical magnitude of the stator to obtain said desired magnitude of power from the stator. generator.

7.

Apparatus according to claim 6, wherein the comparator is adapted to establish, from a

difference between the stator flow reference value and the estimated stator flow value, a stator flow difference vector with a magnitude and direction and said switches operating based on said stator flow difference vector.

8.

Apparatus according to any of claims 6 to 7, wherein said switch control unit (72) is

5 adapted to operate the switches (52, 53) according to a pulse width modulation scheme to generate a voltage waveform synthesized at the stator terminals. 9. An apparatus according to claim 6, wherein the switch control unit (72) is adapted to establish a switching pattern by providing control signals that operate a first set of switches into a state of ignition during a first interval of time,! a, and a second set of switches 10 towards a state of ignition for a second time interval,! B.

10.

Apparatus according to claim 9, wherein the switch control unit (72) is adapted to determine the first and second time intervals according to

eleven.

Computer program product that can be loaded directly into the memory of a device

15 that has digital computing capabilities, comprising portions of software code for performing the steps of any one of claims 1 to 8, when said product is executed by said electronic device.